(Figure 4) and the %foot 0.5% Carbowax column (Figure 5). Preliminary quantitative studies indicate a relatively high conversion of amino acid to derivative. Further studies on the reproducibility of the method, and recovery of Clcamino acid derivatives under various conditions are in progress. Although further work is required before the present procedure can be applied to the complete quantitative analysis of protein hydrolyzates, the method in ita present form is applicable to less difficult analytical problems. For example, it is possible to analyze hydrolyzates of small peptides eluted from paper chromatograms, to detect impurities in commercial samples of amino acids (alloisoleucine in isoleu-
cine, allothreonine in threonine, glycine in @-alanine, hydroxyproline 111 proline, etc.), and to identify the minute amounts of amino acida present in p r e p arations of soluble ribonucleic acida.
(3) Hunter, I.
R. Dimick, K.P., Come' J. W., C 9 m . dt. had. (Londa)1956,294. (4) Lberti A., "Gaa Chromatogre hy 1968" d. H. De& , ed , p 341, &adendo Preaa. New d r k . i9r;k. ----. (6)C Moore, h . 211-&,893Stein, 1954).W. H., J . Bid. ----I
$. A., kmnen. Arthur. Anal. Biochcni. 1,344: (1960). . (7) Spackman D. H., Stein, W. EI., Moore, S., ANAL.CHEM.30, 1185, 1190 (1958). (8) Wevnsnd. F..Ceiner. R.. Chem. Ber. (6) &off,
ACKNOWLEDGMENT
The authors acknowledge the generous support of the National Institutes of Health, Public Health Service, and the National Science Foundation. The senior author is indebted to the National Institutes of Health for a predoctoral research fellowship. LITERATURE CITED
(1) Bayer, E., Reuther, K. H., Born, F., Angno. Chem.69 640 (1967). (2) Bier, M..Teitejbaum, C., Ann. N. Y. Acad..&i.72, 641 (1959).
~
, ANAL. CHEW.31, 1019
(lO)-Zlatkis, A., Oro, J. F., Kimball, A. P., Zbid., 32, 162 (1960).
RECEIVED for review November 29, 1960. Accepted January 30, 1961. Presented
in part before the Division of Biolo 'cal Chemistry, 138th Meeting, AC9, g e w York, N. Y., September 15, 1960.
Analysis of Pyrolyzates of Polystyrene and Poly(methyl Methacrylate) by Gas Chromatography FRANK A. LEHMANN and GERHARD M. BRAUER Dental Research Section, National Bureau of Standards, Washington 25, D.C.
b Analysis of the composition of the pyrolyzates of polystyrene and poly(methyl methacrylate) degraded at temperatures ranging from 400" to 1100" C. was accomplished by gas chromatography. Styrene monomer is the major product in the degradation of polystyrene below 700" C. Poly(methyl methacrylate), below 450" C., depolymerizes nearly exclusively to monomer. Gaseous and liquid products are formed at higher temperatures. Although the results are dependent on the specific conditions employed in the pyrolysis, this rapid method is applicable to the degradation studies of other polymers and shows the relative importance of various pyrolytk reactions over a wide range of temperatures. The procedure also gives valuable information about the structure of polymers and can be used for the determination of polymer constituents.
T
HE RECENT INTEREST in
developing polymers that are heat stable at high temperatures has made it desirable to obtain more information on the degradation reactions and mechanism^ of polymers. A great number of papers have described the thermal behavior of polymers at moderately high temperatures-that is, in the 200" to 500" C. range (3,6). However, relatively little information (8) is available on the reactions occurring a t higher temperatures.
Figure 1. ber
Pyrolysischam-
I
Gas chromatographic techniques that have been employed in the identification of polymers (2,9,4, 9)and quantitative determination of the composition of copolymers (6, 11) also offer a rapid meana for the analyses of volatile pyrolyzates which 80 far have only been made by time-consuming separations or mass spectrometric methods. This investigation deals with the study of the volatile pyrolysis products of polystyrene and poly(methy1 methacrylate) obtained on degradation of these polymers at temperatures ranging from 425"to 1125OC. EXPERIMENTAL
Reagents. The polystyrene was a
thermally polymerized sample of number average molecular weight 233,000. The poly(methy1 methacrylate) was a suspension polymer. Reagent grade liquids and compressed gases were used as standards for the identification of pyrolysis products.
Apparatus and Procedure. A chromatograph with a redesigned inlet system differing from that previously described (11) was used. Figure 1 shows the experimental setup employed to pyrolyze the polymeric material. I n order to expose the specimen to a uniform temperature, to avoid spattering on decomposition, and to determine quantitatively the amount of residue left, the 2- to 3-mg. sample was placed in a weighed high-silica (Vycor) glsse boat (3-mm. outside diameter, 4-mm. height). This material waa chosen to avoid the occurrence of degradation reactions that may be catalyzed by the boat. The boat waa surrounded by a %gage platinum heating coil having a total resistance of about 0.2 ohm. The ends of the coil were silver-soldered to two copper wire leads connected to a series of variable transformers by which the temperature of the boat could be regulated. The pyrolysis temperature was measured by placing a ChromelAlumel thermocouple (gage. No. 42 wire) in a small indentation in the VOL 33, NO. 6, MAY 1961
673
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E J
i
i
t
i
0
I
I
3
4
6
TIME
b
- i7
i I
t
s
t IO
ii
II
12
IN MINUTES
Figure 3. Chromatograms of gaseous products of polystyrene
pyrolysis
Column, dllco g$i SOL tomp., 100' Cj Aow rate, 60 ml/mkr P y d y d r tomp, C Top. 1025 Bottom. 8 2 5 (Attonuatlon sealo Indlcotod by numborr In tlguro)
termine the presence of any nonvolatile reaidue. A series of known amounts of monomera was also introduced into the column. The peak areaa of the result-. ing chromato ram8 served as etanderd CUNM for t i e determination of the amounts of product present in the p rolyzates and for d e t e m i d q t e amounts of volatilea retained rn the column.
K-
Figure 2. Chromatograms of pyrolysls products of polystyrene Column, Aplero; 11 col. temp., 140' C.1 Aow roto, 60 mL/mln Pyrolyrls tomp, C Top. 1025 Mlddlo. 8 2 5 bottom.425 (Attonuatton r a l o Indlcotod by numbon in flguro)
bottom of the boat. This thermccouple was calibrated egainst a second thermocouple placed m t h the sample inside the boat. During the actual pyrolysis runa this second thermocouple was removed from the boat to prevent any catalytic effects on the polymer d e gradation due to the metals of this thermocouple. Measurements of the pyrolysis temperature were considered to be accurate within 10' C. The copper wire leads were sealed into a standard taper (14/35) polytetrafluoroethylene plug which was inserted into a female glaa joint. This
Table 1. Operating Conditions Column length, 6 feet Bore (I.D.), 1/18 inch Column materiels, 30% (wt.) A i e ~ o n L,Chlorowax 70,and dinonyl phtialate on ground firebrick 30-60 mwh; eilica gel 30-200 mesh Carrier gas, helium Flow rete, 20 60 ml./.min. Preseure heed, 20 p.e.1. Column temperatures, loo", IN",,'%I or 140' C. Sainple quantity, 2 to 3 mg.
674
0
ANALYTICAL CHEMISTRY
joint was connected to the metal inlet s stem of the chromatograph by two *metal d. After fiushing the apparatus with helium for at least 3 minutes an electric current was passed through the heating coil. The ,voltage of the transformers was preset to heat the boat to the d e sired temperature which was reached within 15 seconda at 425' to 825' C. and within 3 to 6 seconds at the higher temperatures. the temperature was maintained /or 20 to 30 seconds a t 425' to 825O C. and for 5 to 10 seconds a t 825' to 1125' C. The helium p d g over the sample carried away the vaporized products into the column. The column material and temperature were varied until chromatograms with sharp pesh that did not overlap and suitable retention times were obtained. of the experimental con& k x x d i n g the column materials and temperatures as well as the carrier gas flow rate and pressure is given in Table I. The column was considered to be purged of the more volatile pyrolytic products when no further peaka on the chromatogram were obtained for at least 10 minutes. The boat waa removed from the chromatograph, cooled, snd weighed to de-
$ss-
RESULTS
Polystyrene. Typical chromatograms obtained on pyrolyzing polystyrene at 425', 826', and 1025' C. are shown in Figure 2. At 425' C. only styrene monomer is eluted from the column; degradation a t 826' C. produces a number of identifiable products. Reactions leading to these products are even more important at a temperature of 1025' C. Identification of the compounds from the various peaks was made by comparing their retention times with those obtained from chromatograms of known compounds. The composition was not altered appreciably on increaaing the temperature from 1025" to 1125' C. as evidenced by the nearly identical chromatogram for the pyrolyzates. On heating the boat above 1025" C. the polymer was probably flash-pyrolyzed completely before the boat reached this temperature. The initial peak for the gaseous products could not be resolved by columns containing hydrocarbon high-vacuum grease (Apiezon L) or chlorinated p a r f f i wax on ground firebrick (Chlorowax 70). Chromatograms with a silica gel column showed the presence of ethylene, acetylene, and some carbon dioxide (Figure 3). Results of the analyses a t various pyrolysis temperatures are shown in Table 11. Duplicate runs usually agreed
Table II.
Cornposition, Weight Per Cent, of Pyrolyzates of Polystyrene Chlorowax 70 on ound [Average of two or more analyses. Column: 30% (p.) brick, 30-60 mesh (except where indicated otherme); col. temp., 140"
Pyrolysis Temperature, " Centigrade Product Carbon dioxide Ethylene Acetylene Benzene Toluene 60 400
I 500
I
6oD
I 100
1 800
I 900
I
1000
1100
PYROCYSIS TEMPERATURE,' C.
Figure 4. Effect of degradation temperature on amount of styrene monomer in pyrolyzates
425
... ...
... ...
525 I
.
625
725
... ...
.
... ...
T&e8 Tracea Trace
... ...
...
825
...
4.1. 4.1" 8.1
1025
(?]
1026
Trace" Trace" 6.9" 6.9" 13.4
6.9. 6.7" 12.0* 2.7'
Ere-
1125 6.8" 6.8. 13.0
Tracd Trace' Trace' 0.9. 2.5O 5.6* 6.8' Ethylbenzene 3.7' Styrene 64.3 67.5 74.4 83.9 73.7 62.8 63.8 64.3 Material retained in column 35.7 32.6 26.6 14.4 7.5 4.3 4.3 3.3 Values obtained by resolving a Column: silica gel 30-200 meah; col. temp., 100' C. totel percentage of gaseous producta with Chlorowax 70 and Apiezon L column, respectivbe801umn: 30% (wt.) Apiezon L on ground firebrick, 30-60 mesh; col. temp., 140' C. a Weight per cent of toluene and ethylbenzene.
within 1%. Substitution of an Apiemn L for the Chlorowax 70 column gave chromatograms with peaks a t different retention times, but did not change appreciably the results of the chromatographic analyses. The Apiemn L column, however, separated the ethylbenzene-toluene single peak obtained with Chlorowax 70. In all degradations a t temperatures above 350" C. no residue was found on reweighing the boat after cooling. However, a t 425" C., 35.7% of the volatile material is retained in the column. Based on the results of previous studies (7, IO), this is mainly styrene dimer. The quantity of material retained in the column decreases with the pyrolysis temperature and amounts to only 4.3% a t 1025" C. Percentage of styrene monomer in the pyrolyzate us. pyrolysis temperature is shown in Figure 4. At 425" C., 64.3% monomer is present in the pyrolyzate. The monomer content increases on raising the degradation temperature and reaches a ma.ximum around 725" C. Reactions leading to the formation of ethylbenzene, toluene, benzene, ethylene, and acetylene occur at higher temperatures and the percentage of styrene in the pyrolyzate decreases. Variations of the flow rate of the camer gas between 45 and 75 ml. per minute did not alter the composition of the degradation products. At lower flow rates (less than 45 ml. per minute) the primary products were not immediately camed away from the hot zone and secondary reactions probably took place. Thus, the percentage of benzene was increased more than twofold in the pyrolysis a t 1025" C. when the flow rate was reduced from 60 to 10 ml. per millUte. Comparison of the chromatographic resulta with mass spectrometric analyees of polystyrene degraded in a helium atmosphere by Madorsky and Straus (8) indicates that consistently larger quantities of monomer are obtained by the chromatographic procedure. The high flow rate of the carrier
Table 111.
Composition, Weight Per Cent, of Pyrolysis Products of Poly(methy1 Methacrylate)
[Column: 30% (wt.) dinonyl phthalate on
1280.6
ound firebrick 30-60 mesh; col. temp.,
Pyrolysis Temperature, ' Centigrade Product 426 625 626 726 825 875 Caseouscomponenta Trace 3.6 4.2 18.2 60.3 73.6 Methanol ... ... Trace Trace Trace Trace Ethanol ... Trace Trace 0.6 0.5 Methyl acrylate TrLe Trace Trace Trace Trace Methyl propionate Trace Trace 0.4 0.5 0.7 0.2 Methyl lsobutyrate Trace Trace 0.4 0.6 0.8 2.5 Methyl methacrylate 99.4 96.2 94.7 80.6 37.4 23.2 Reaidue Nil NI1 Nil Nil Nil Nil Column temperature, 100' C.
TIME
Figure 5.
N
1026. 76.3
Trace 0.4
Trace 0.3 3.0 19.9
Nll
MINUTES
Chromatograms of pyrolysis products of poly(methy1 methacrylate)
Cdmn, dlnonyl phthalato Top. Pyrolysistamp., 425. CJcd temp., 128' CJHo b w roto, 60 mL/mln I)onom. Pyrolysis tomp., 1025' 4d tamp., 100' C, Ho Aow rate, 20 ml/rnin. (Altonuation r o b indkatod by n m b o n In flgura)
VOL 33, NO. 6, M A Y 1961
675
Table IV. Composition, Weight Per Cent, of Gaseous Pyrolyzates of Poly(methy1Methacrylate) (Column: silica gel, 30-200 mesh; col. temp., 100' C.) Pyrolysis Temp., 'C: Product 825 1025
+
Methane carbon monoxide 47.9 12.4 Ethane 8.4 2.2 Carbon dioxide 18.5 42.6 Ethylene 12.6 24.9 Acetylene 12.6 17.9 At 425' C., only trace values were
found.
Table V. Elementary Composition of Polymer and Degradation Products (Pyrolyaia temperat$ure,1025' C.) Polvstvrene _ go-ljum& . -
c, % H,%
0,%
92.2 7.8
Pyrolysis producte 91.8 8 . 2 T r i e Poly(methyl methicrylate) Polymer 60.0 8.0 32.0 Pyrolysia producte 69.4 7.4 33.2
gas rapidly sweeps the primary reaction products from the hot zone into the chromatographic column. On the other hand, the pyrolysis chamber used by Madoraky and Straus is quite large, and the primary products are kept at the elevated temperature for a considerable time period, during which they can undergo secondary decompositions. Poly(methy1 Methacrylate), Chromatograms obtained from pyrolyzing poly(methy1 methacrylate) a t 425' end 1025' C.using a dinonyl phthalate column are shown in Figure 5. Monomer is formed nearly exclusively a t 425' C., whereas a number of compounds are detected a t 1025' C. NO volatile products are retained in the column. The gaseous products which appear as a single peak on the chromatograms are separated and identified with a silica gel-packed column, with the exception of methane and carbon monoxide, which give overlapping peaks. Tables I11 and IV give the composition of the pyrolyzates a t degradation temperatures from 425' to 1025" C. (Values listed are the result of two analyses.) Figure 6 shows the percentage of monomer product as a function of degradation temperature. At 425' C. the product contains 99.4% monomer. The percentage of methyl methacrylate monomer in the pyrolyzate decrenses to about 20% a t 875' C. and remains constant a t higher temperatures. Gaseous products predominate at 825" C. and above. On raising the temperature from 825' C. to 1025' C. the percentage of carbon dioxide, ethylene, and acetylene in the gaseoua mixture increases, whereas the percent676
ANALYTICAL CHEMISTRY
3 :[ to 101
I
6
1
I
W
I
~
L I
7
I I e
mo
O
PYROL'ISIS TELPERATUK,*C.
Figure 6. Effect of degradation temperature on amount of methyl methacrylate monomer in pyrolyzate
age of methane, ethane, and carbon mon-
oxide is lowered. DISCUSSION
Products obtained on pyrolysis of polystyrene and poly(methy1 methacrylate) between 425' and 1025" C. can be analyzed readily by chromatographic techniques. Table V shows the compariaon of the percentages of carbon, hydrogen, and oxygen in the polymer a m p l e and in the products of the pyrolysis. The nearly identical percentages of carbon, hydrogen, and oxygen obtained for the pyrolysis products and those calculated for the original polymer indicate that any volatile materiels retained in the column, such as is observed on degradation of polystyrene, are mainly polymer fragments. The chromatograph employed a filamenbtype katharometer for measuring the thermal conductivity of the effluent gas. Corrections for variations of the thermal conductivitiea of the vaporized liquid components of the pyrolysis mixture appear unneceassry. Results accurate within 1% were obtained for synthetic mixtures having the same composition as the liquid pyrolyzates. This error is of the same order as that for the reproducibility of duplicate determinations of the composition of the pyrolyzate. In degradations in which the percentages of gaseous products are large and their conductivity values at the operating temperature of the detector are available, corrections for variations of the conductivities are recommended. At temperatures below 350' C. degradation is still occurring after the retention times of the pyrolysis products. Hence, the chromatogram will show taiiing peaks which make it difficult to determine quantitatively the compo&tion of the pyrolyzate. Similar 'to other techniques that are based on the behavior of polymers on heating, the results are dependent on the experimental conditions employed in the degradation. Analyses of the decomposition products of polystyrene show that faster carrier gaa flow rates
and therefore more rapid removal of the products from the hot reaction zone produce increased proportions of monomer. Since the reactions do not proceed to equilibrium, their extent depends on the initial reaction temperature and is influenced by the rate of cooling of the products. The latter variable is influenced by the temperature and rate of flow of the carrier gas as well ~ ~ as by the geometry and thermal properties of the pyrolysis vessel. It is not possible to state the complex reactions which lead to the products ob~ ~ without e ddetailed kinetic studies. Inspection of Tables 11, 111, and IV shows that reaction products other than the respective monomers are found in increasing amounts as the temperature of the pyrolysis is increased. The C a r -C bond of styrene and the bonds adjacent to the C=C in methyl methacrylate monomer are probably most susceptible to cleavage. The resulting radicals may undergo a variety of reactions such as disproportionation, further cleavage of bonds, or recombination with other fragments. Free radicals formed on scission of the CabC bond will yield benzene and acetylene. Some of the acetylene as well as some of the remaining styrene may react with hydrogen, another possible product, to form ethylene and ethylbenzene. Secondary decomposition reactions of methyl methacrylate are even more complex and much work remains to be done to clarify the intermediate steps. ACKNOWLEDGMENT
The authors thank A. F. Forriati and John Strassburger for their valuable suggestions. LITERATURE CITED
(1) Davison, W. H. T., Slaney, S., Wragg, A. L., Chem.&Id. (Lonflon) 1954,1136. (2) Grassie, N., 'Themutry of High
Polymer Degradation Processes,'' Chap. 11, pp. 24-116, Intemience, New York, 1956. (3) Guillet, J. E., Wooten, W. C., Combs, R. L.. J. A .. m l . Polvmer Sci. 3, 61
( 1960): (4) Hsslam, J., Jeffs, A. R., J . A p p l . Chem. 7, 24 (1957). (5) Jellinek, H. ,,H., "Degradation of VmylPolymera, Chap. I1 pp. 65-180, Academic Press, New York, 1955. (6) Lehrle, R. S., Robb, J. C., Nalurc 183, 1671 (1959). (7) Madorsky, S. L., Straus, S., J. ReS e O T e h Nail. BUT. S.!~&T& 40, 417 (1948). (8) Zbid.,63A,261 (1959). (9) &dell, E. A., Strutz, H. C., ANAL. CBM. 31, 1890 (1959). (10) Staudinger, H., Steinhofer, A., Ann. 517, 35 (1935). (11) Straaaburger, J:, Brauer, G. M.,
Tryon, M., Forzlati, A. F., ANAL. CEEM.32, 454 (1960). RECEIVED December 20, 1960. Accepted March 3, 1961. Presented in part, Diviaion of Pol mer Chemist 138th Meeting, ACS, d e w York, N.?., September 1960.
